Configurations and methods of co2 capture from flue gas by cryogenic desublimation

ABSTRACT

Systems and methods of CO 2  desublimation are presented in which refrigeration content is retained within the system. Most preferably, refrigeration content is recycled by providing the refrigeration content of a CO 2 -lean feed gas to the CO 2 -containing feed gas and to pre-cooling of a desublimator, and/or by providing refrigeration of effluent of a desublimator in regeneration to a refrigerant in a closed refrigeration cycle for deep-cooling of another desublimator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority benefit under 35U.S.C. §121 to co-pending U.S. patent application Ser. No. 13/940,019,filed Jul. 11, 2013, and entitled “CONFIGURATIONS AND METHODS OF CO2CAPTURE FROM FLUE GAS BY CRYOGENIC DESUBLIMATION”, which claims priorityto U.S. Provisional Patent Application Ser. No. 61/670,427, which wasfiled on Jul. 11, 2012, and entitled “CAPTURE OF CO2 FROM A FLUE GAS BYCRYOGENIC DESUBLIMATION”, both of which are incorporated by referenceherein in their entirety.

FIELD OF THE INVENTION

The field of the invention is systems and methods of CO₂ desubliminationfrom flue gas and recovery thereof.

BACKGROUND OF THE INVENTION

The increasing carbon dioxide concentration in the atmosphere issuggested to be linked to increasing temperatures and changes in theWorld's climate, and significant interest has been given to CO₂ capture,especially from combustion gases. One well established CO₂ capturemethod uses a chemical solvent (typically amine based) to absorb CO₂from flue gas. This method is effective, however presents severaldifficulties: (1) solvents must be manufactured, purchased, andtransported, resulting in significant expenses; (2) most solvents (andespecially amine solvents) tend to degrade via oxidative and/or thermalpathways. As a consequence the solvent must be maintained and/orcontinuously replaced; and (3) some solvents present environmentalconcerns as they may form harmful compounds once emitted to theatmosphere.

Other known CO₂ capture processes may circumvent at least some of thesedrawbacks, including PSA (pressure swing adsorption) or membraneprocesses. However, these processes are often costly because they carryenergy penalties caused by the regeneration of CO₂ loaded adsorbents orby the need for recompression of flue gas upstream of a membrane.

CO₂ can also be removed from a gas stream based on the principles ofcryogenic removal or desublimation. For example, U.S. Pat. Pub. No.2011/0226010 teaches separation of CO₂ from flue gas by compression,expansion and refrigeration. While such process is at least conceptuallyattractive, significant energy requirements often render such separationimpracticable. This and all other extrinsic materials discussed hereinare incorporated by reference in their entirety. Also, where adefinition or use of a term in an incorporated reference is inconsistentor contrary to the definition of that term provided herein, thedefinition of that term provided herein applies and the definition ofthat term in the reference does not apply.

In another example, as described in U.S. Pat. No. 4,265,088, CO₂ isfrozen out from a gas stream, and so isolated solid CO₂ is thensublimated after it has been deposited in a tower. Such configuration,however, is typically not able to generate a pure CO₂ stream andrequires in most cases substantial quantities of energy for CO₂recovery. Similarly, U.S. Pub. No. 2011/0023537 teaches desublimation ofCO₂ on porous media and CO₂ recovery via fluid CO₂ to so produce a warmporous medium. However, use of such porous media may be problematic dueto potential clogging. Furthermore, CO₂ recovery using liquid CO₂ asdescribed in the '537 reference is not energy efficient under variouscircumstances.

Thus, there is still a need for energy efficient systems and methods ofcapture of CO₂ from a flue gas by cryogenic desublimation, and a furtherneed for the removal of the solid CO₂.

SUMMARY OF THE INVENTION

The present invention is directed to various configurations and methodsof CO₂ removal with desublimation in which refrigeration content in thedesublimation system is recycled within a CO₂ recovery plant andretained to a significant degree. Most preferably, contemplated plantsoperate with a plurality of desublimators that are fluidly coupled viavalves and a control unit such as to allow each desubliminator tooperate in one of the following modes: a desublimation mode, aregeneration mode, a pre-cooling mode, and a deep-cooling mode.

In one preferred aspect of the inventive subject matter, a flue gastreatment plant includes a flue gas conditioning unit that cools fluegas in one or more heat exchangers, preferably using a cool CO₂ depletedflue gas to so produce a precooled flue gas at a low flue gas pressure.Contemplated plants further include a desublimation unit having multipledesublimators that are fluidly coupled to each other and the flue gasconditioning unit such that: (a) one desublimator receives the precooledflue gas from the flue gas conditioning unit and produces a coldCO₂-lean flue gas while solid CO₂ deposit in the desublimator; (b)another desublimator receives and is pre-cooled by the cold CO₂-leanflue gas and so forms the cool CO₂ depleted flue gas; (c) a furtherdesublimator receives a warm liquid CO₂ stream at relatively highregeneration pressure to produce an effluent stream that includes atleast some of the desublimated CO₂ (preferably in slurry form); and (d)yet another desublimator deep-cooled by a refrigeration cycle to atemperature at which CO₂ desublimates; and wherein the refrigerationcycle is thermally coupled to the effluent stream such thatrefrigeration content of the effluent stream cools a refrigerant in therefrigeration cycle.

It is further typically preferred that the flue gas conditioning unitalso includes a second heat exchanger that cools the flue gas withresidual refrigeration content of the cool CO₂ depleted flue gas leavingthe first heat exchanger. Where desired or needed, the flue gasconditioning unit may further comprise a dehydration unit that removeswater from the flue gas. Particularly suitable refrigeration cycles areclosed refrigeration cycles, or semi-open cycle refrigeration cyclesthat use a portion of the cool CO₂ depleted flue gas as a refrigerant.Regardless of the nature of the cycle, it is also preferred that therefrigeration cycle includes a cross-heat exchanger that further coolsthe pressurized cooled refrigerant using refrigerant leaving thedesublimator that is in deep-cooling mode.

Most typically, one or more of the desublimators contains structuredpacking, random packing, or a non-porous high surface area material.Additionally, it is preferred that the flue gas pressure is between 10and 50 psia while the regeneration pressure is between 100-300 psia.Particularly preferred flue gas treatment plants will include a controlsystem and a series of valves that are fluidly coupled to the pluralityof desublimators to allow switching of an operational mode of at leastone of the desublimators from a desublimation mode, a regeneration mode,a pre-cooling mode, or a deep-cooling mode to another one of thedesublimation mode, the regeneration mode, the pre-cooling mode, and thedeep-cooling mode.

Therefore, and viewed from a different perspective, the inventors alsocontemplate a method of treating flue gas in a flue gas treatment plantthat includes a step of using a first desublimator to receive aprecooled flue gas, and to produce solid CO₂ and a cold CO₂-lean fluegas (107) using desublimation. In another step, the cold CO₂-lean fluegas (107) is used to pre-cool a second desublimator to a temperatureabove desublimation temperature for CO₂, thereby forming a coolCO₂-depleted flue gas (108), and in yet another step, residualrefrigeration content of the cool CO₂-depleted flue gas is used to coola feed gas (101) to thereby form the precooled flue gas (106), whereinthe first desublimator is deep-cooled using a refrigerant of arefrigeration cycle before the step of using the first desublimator, andwherein the refrigeration cycle is thermally coupled to a heat exchangerthat cools the refrigerant in the refrigeration cycle usingrefrigeration content of stream within the flue gas treatment plant.

In especially preferred methods, the stream within the flue gastreatment plant is an effluent of a third desublimator, and mostpreferably, the effluent is a two-phase stream comprising liquid CO₂ andsolid CO₂. It is still further preferred that solid CO₂ is removed fromthe first desublimator using a liquid CO₂ stream at a pressure andtemperature (e.g., 100-300 psia; −10 to 40° C.) that does not allow forformation of gaseous CO₂. While not limiting to the inventive subjectmatter, the first desublimator is operated in at least some embodimentsat a pressure of between 10-50 psia, and the flue gas and a stack gasleaving the flue gas treatment plant have a temperature of between10-40° C.

Therefore, the inventors also contemplate a method of recyclingrefrigeration content in a desublimation flue gas treatment plant inwhich flue gas and a second desublimator are pre-cooled usingrefrigeration content of a cold CO₂-lean flue gas that leaves adesublimator to which the precooled feed gas (106) is fed. In anotherstep, and before the first desublimator is operated in desublimationmode, the first desublimator is deep-cooled using a refrigerant of arefrigeration cycle, wherein the refrigerant is cooled by anotherdesublimator effluent in the flue gas treatment plant (and mostpreferably an effluent of a desublimator in regeneration mode).

In especially preferred methods, the flue gas is pm-cooled in twoseparate heat exchangers, with the first heat exchanger cooling the feedgas to a temperature above 0° C., and the second heat exchanger coolingthe feed gas to a temperature above a desublimation temperature for CO₂.Where desired or needed, the flue gas may be subjected to a dehydrationstep.

Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are one exemplary configuration of CO₂ removal from aflue gas with integrated cold recovery.

FIGS. 2A and 2B are the configuration of FIG. 1 with flue gascompression/expansion.

DETAILED DESCRIPTION

The inventive subject matter is directed to various systems and methodsfor CO₂ removal from flue gas using desublimation that are particularlyeffective in the recovery of refrigeration content from streams withinthe system.

In general, contemplated systems and methods allow for CO₂ separationfrom flue gas as a result of differences in intrinsic thermodynamicproperties between CO₂ and other components in the flue gas. Morespecifically, the inventors have developed systems and methods tocapture solid CO₂ through desublimation at relatively low pressure, andrecovery of the solid CO₂ via use of liquid CO₂ at relatively highpressure.

As the process of CO₂ desublimation is energy demanding, the inventorshave developed systems and methods to recycle refrigeration content thatallows substantially more economical operation. In one particularlypreferred aspect, thermal integration between the flue gas entering thedesublimators and the CO₂-lean effluent gas, and thermal integrationbetween a deep cooling refrigeration cycle and a liquid CO₂ recoverystream provide substantial advantages as compared to heretofore knowncryogenic processes. As a result, most or all of the input and outputstreams can be at about ambient temperature (e.g., 20° C., ±10° C.) as alarge fraction of the refrigeration is recycled within the system.

In particularly preferred aspects of the inventive subject matter,desublimation of CO₂ is performed using a plurality of desublimationcolumns that are operated such that continuous pre-cooling,deep-cooling, desublimation, and/or regeneration for respectivedesublimation columns can be performed. As used herein, the terms“desublimation column” and “desublimator” are used interchangeably anddenote the same device. Most typically, but not necessarily, systems andmethods contemplated herein include at least four columns and associatedpiping such that one column can be operated as desublimator while theother columns can be subjected to pre-cooling, deep-cooling, andregeneration, respectively. It is moreover generally preferred that thesystems and methods provided herein employ flue gas conditioning toremove water and reduce the temperature of the flue gas prior to entryof the cooled flue gas into the desublimator. As will be explained inmore detail below, flue gas conditioning as well as pre-cooling isadvantageously performed using residual refrigeration content of theflue gas exiting the desublimation column. To even further economizeoperation, refrigeration content of desublimated CO₂ is used to cool(partially or even entirely condense) a refrigerant of a preferablyclosed refrigeration cycle that is configured to deep-cool adesublimation column in preparation for desublimation.

With respect to flue gas conditioning it is generally preferred that theflue gas is boosted or pressurized and cooled prior to entering adesublimator, preferably using a booster/blower or a compressor. Mostadvantageously, cooling of the flue gas is accomplished via heatexchange with CO₂-lean flue gas produced by a desublimator (or apre-cooling desublimator) and/or an external refrigerant. Optionally,refrigeration content may be derived from expansion of CO₂-lean flue gasas also further detailed below. Most typically, a dryer may be used toremove at least a portion (and preferably substantially all) of thewater content (and other condensable components) from the flue gas.However, it should be noted that in a further aspect of the inventivesubject matter, the flue gas is not completely dried before it entersthe desublimator. In this case, it is preferred that water from the fluegas is mostly removed by cooling the flue gas below the dew point,followed by removal of the water in a subsequent knock-out drum. Thewater that remains in the flue gas will then desublimate in the CO₂desublimator. The solid water is then melted by the warm CO₂ liquid, isentrained in the desublimator effluent stream, and can be recovered, forexample, by a liquid dryer.

It should further be noted that the flue gas refrigeration willpreferably be performed in multiple stages, with a first stage coolingthe flue gas to a temperature above the freezing point of water (i.e.,above 0° C.), which advantageously allows condensation of most of thewater contained in the flue gas. A second stage can then be used tochill the dehydrated feed gas to a temperature below the freezing pointof water but above the desublimation point of CO₂ (i.e., below 0° C. andabove −100° C., more typically above −115° C. at about atmosphericpressure) Residual water content can be removed in various manners,including molecular sieves or glycol driers. Still further, it should berecognized that flue gas cooling is most preferably performed usingrefrigeration content from within the CO₂ desublimation system. Thus,suitable sources of refrigeration content include the CO₂-lean flue gas,refrigerant of the closed cycle refrigeration loop for deep-cooling, andCO₂ liquid for regeneration, preferably where the liquid is a two-phaseliquid.

Most typically, the flue gas is a combustion gas of combustor of aturbine in a power production plant, but may also be a combustion gasfrom one or more boilers, heaters, or even exhaust gas from anincinerator or catalyst regenerator. Thus, the nature of suitable fluegases may vary considerably. However, it is generally preferred that theflue gas is produced in relatively significant quantities, for example,at a rate of at least 10 scfm, more typically at least 1,000 scfm, andmost typically at least 10,000-100,000 scfm, and even higher. Based onthe nature of the flue gas and prior treatment (e.g., desulfurization,NO_(x) removal, particulate removal, etc.), chemical composition andtemperature of the flue gas vary considerably. Thus, in most aspects ofthe inventive subject matter, the flue gas is pre-treated to remove oneor more undesirable components (e.g., SO_(x), NO_(x), Hg, ash, otherparticulates, etc.). In yet further contemplated aspects, it ispreferred that the flue gas will have a temperature of less than 200°C., more preferably less than 100° C., even more preferably less than50° C., and most preferably less than 30° C., Especially preferred fluegas temperatures will thus be in the range of between 10-50° C., 20-70°C., 30-80° C., or 15-90° C.

Suitable CO₂ levels in the flue gas may also vary. However, it isgenerally preferred that the CO₂ concentration in the flue gas isbetween 0.1-2.0 vol %, between 2.0-5.0 vol %, between 5.0-20 vol %, andless typically between 20-50 vol %. Most typically, the CO₂concentration in the flue gas is between 5-20 vol %, or between 10-25vol %. It should still further be appreciated that where the flue gaswas not subject to pretreatment, such pretreatment may be performed in aseparate desublimator. For example, SO_(x) or NO_(x) removal may becarried out by desublimation of SO_(x) or NO_(x) components prior todesublimation of CO₂.

Where the flue gas is provided by a flue gas source that is sensitive toback pressure (e.g., turbine combustor), it is generally preferred thatthe flue gas is boosted to a pressure that is at least the back-pressureof the downstream components (e.g., desublimation columns and heatexchangers), typically between 15-30 psia, more typically between 30-50psia, and in some cases between 50-150 psia. Where desublimation isperformed at elevated pressures, the flue gas may be compressed to suchpressures, typically between 25 and 125 psia, as described in moredetail below.

It should be appreciated that desublimation is a process of transforminggaseous CO₂ into solid CO₂ without undergoing a liquid phasetransformation. Desublimation is achieved in most typical cases usinglow pressure and temperature conditions, and the person of ordinaryskill in the art will readily be apprised of suitable desublimationconditions with reference to phase diagrams well known in the art (e.g.,2D or 3D phase diagrams, temperature/entropy diagram, pressure enthalpydiagram, etc.). For example, desublimation temperatures for CO2quantities typically encountered in flue gases will generally be below−90° C., more typically below −100° C., even more typically below −115°C., and most typically below −130° C. where the flue gas pressure isbetween 15-30 psia. At elevated pressures, the desublimation temperaturewill rise as can be readily taken from known phase diagrams.

With respect to desublimation devices it is generally contemplated thatat least one, but more typically more than one desublimators are used tocapture the CO₂ from the flue gas and recover the CO₂ as solid CO₂. Itis still further particularly preferred that the desublimators contain astructured packing to more effectively desublimate CO₂ and/or to allowfor facile regeneration using liquid CO₂, However, numerous alternativepacking materials that increase the surface area are also contemplatedherein, and especially non-porous random packing materials. Porouspacking materials are generally less preferred and in most instanceseven excluded.

Notably, and with further reference to known phase diagrams, CO₂ willonly desublimate (and not liquefy) at sufficiently low temperatures andlow partial pressure, while solid CO₂ can be recovered at a later pointas liquid CO₂ (or as a solid/liquid slurry) at higher pressures andtemperatures without generating gaseous CO₂. In one especially preferredmanner of operation, multiple desublimators are operated in acoordinated cycle in which one desublimator is used for desublimation,another for pre-cooling, a further for recovery of the solid CO₂, andyet another one for deep-cooling. Seamless operation of thedesublimators may be achieved by use of valves and suitably configuredcontrol circuits, wherein the valves open and close with respect to thefunction of the desublimator (i.e., whether the desublimator isdesublimating, deep-cooling, recovering CO₂, or pre-cooling).

Furthermore, although the steps in the systems and methods are discussedin a particular order, various alternative sequences and numbers ofdesublimators are also deemed suitable. For example, it is contemplatedthat the exit stream of the desublimator which is desublimating the CO₂from the flue gas enters the deep-cooling desublimator so that more CO₂is extracted, or that two separate desublimators are performing the samefunction of CO₂ removal. Likewise, pre-cooling and/or regeneration maybe performed on more than one desublimator at a time. Most typically,desublimation is ended in a particular desublimator upon recovery of apredetermined quantity of desublimated CO₂, or upon consumption ofavailable cooling necessary for the desublimation of CO₂.

Due to the relatively high refrigeration demand for desublimation, it ispreferred that the desublimator is cooled in a sequence of steps thatinclude at least one pre-cooling step in which residual refrigerationcontent from cold CO₂-lean flue gas is used as a pre-cooling medium.However, it is contemplated that other process streams may also be used,either in direct contact, or via a heat exchanger and/or heat exchangefluid. Thus, external refrigeration is also deemed suitable to pre-coolthe desublimator. Such external cooling may be particularly advantageouswhere the flue gas is produced from a gas combustion turbine, and wherethe gas is derived from LNG. For example, regasification of the LNG maybe at least in part performed by using the refrigeration content of LNGto pre-/deep-cool the desublimation column, and the so heated LNG may befurther warmed prior to combustion (which then produces the CO₂containing flue gas). Depending on the flow rate and temperaturedemands, it should be appreciated that the CO₂-lean flue (or other) gasexiting the pre-cooling desublimator may be further used to cool theflue gas in the flue gas conditioning unit before it enters thedesublimator. In addition, the CO₂-lean flue (or other) gas may be usedto cool any other stream in the process until it has reachedapproximately ambient temperature (typically 20° C., +1-10° C.). Usingthe CO₂-lean flue gas to cool streams in the process is more energyefficient since the CO₂-lean flue gas is already at a coolertemperature. Therefore, the net result is a conservation of energy,which is highly desirable.

With respect to deep-cooling of a pre-cooled desublimation column, itshould be noted that any refrigerant may be used to deep-cool thedesublimator and its packing to a temperature suitable to capture CO₂ bydesublimation. However, it is typically preferred that the refrigerationcycle for deep-cooling is a closed refrigeration cycle that is thermallyintegrated with the regeneration cycle, and especially with the effluentstream of a regenerating desublimator such that the refrigerationcontent is conserved within the system. For example, warm pressurizedrefrigerant of the refrigeration cycle may be cooled by heat exchangewith a CO₂ slurry mixture from the bottom of a regeneratingdesublimator, and further cooled by the refrigeration content of arefrigerant stream exiting the desublimator as shown in more detailbelow. Therefore, it should be recognized that it is possible tosubstantially reduce the need for external energy in the deep-coolingrefrigeration and CO₂ regeneration operations.

In regeneration, recovery of solid CO₂ is preferably achieved by use ofliquid CO₂ at high pressure (e.g., between 50-250 psia, between 100-300psia, or between 250-500 psia) and preferably about ambient temperature(e.g., between 5° C. and 40° C.). However, it is noted that lowertemperatures are also deemed suitable. Using liquid CO2 at such pressureand temperature, a portion of the deposited, solid CO₂ fromdesublimation is transformed into a liquid phase CO₂ (and in most casesnot to gaseous CO₂). It should be appreciated that once the desublimatorhas achieved the requisite high pressure to transform a portion of thesolid CO₂ into liquid CO₂, the liquid CO₂ stream can be continuouslypumped into the desublimator to recover additional solid CO₂. Thus,particularly preferred operating conditions for regeneration of thedesublimator are conditions at which CO₂ can exist in the liquid and/orsolid state but not in the gaseous state.

Therefore, it is generally preferred that the effluent stream of thedesublimator during regeneration is a CO₂ slurry (i.e., a two-phasesystem comprising solid and liquid CO₂). Most preferably, the effluentstream may exchange heat with a portion of the deep-cooling refrigerantstream such that the effluent stream is heated to melt the slurry (i.e.,to reduce or eliminate solid CO₂ from the slurry to thereby form athinner slurry or single-phase CO₂ liquid) and such that therefrigeration content from the effluent stream remains within the systemby recycling the refrigerant content to the deep-cooling operation.Additionally, one may also choose to heat the effluent stream usinganother (preferably waste heat) stream from the process in a heatexchanger or using an external energy source. Once enough liquid CO₂ iscollected, one may choose to purge a portion out of the system by use ofa pump to further processing, sale, or final disposition (e.g.,sequestration).

FIG. 1 is one exemplary process schematic according to the inventivesubject matter. Here, flue gas 101 (typically desulfurized) enters ablower BL-101 or other device to increase pressure of the flue gas. Theflue gas typically has a relatively low concentration of CO₂, up toapproximately 18 vol %. The blower generates a pressurized flue gasstream 102, with a pressure high enough to overcome the pressure drop ofthe downstream system components to so avoid backpressure on the sourceof the flue gas.

Pressurized flue gas stream 102 enters first precooler E-101 that ispreferably configured as a heat exchanger in which refrigeration contentof the cold, CO₂ depleted flue gas 109 is used to cool the pressurizedflue gas stream 102, thereby forming cooled pressurized flue gas stream103. Stream 103 is cooled and dried in dryer D-101, which may be aglycol or other suitable gas dryer. Water is removed from the cooledpressurized flue gas as stream 105, forming dry cooled flue gas 104,which enters a second precooler E-102. The second precooler E-102 ispreferably configured as a feed-effluent exchanger in which heat istransferred from the dry cooled flue gas 104 to the cool CO₂ depletedflue gas 108. The so precooled dry flue gas 106 enters one of a seriesof desublimators (C-101 A-D).

In the example of FIG. 1, the precooled dry flue gas 106 enters thefirst desublimator (C-101 A), However, it should be recognized that theprecooled dry flue gas could enter any of the four desublimators,depending on the cycling service. Moreover, it should be noted that theflow path in the desublimator can be radial or axial, and that asuitable flow path will be readily determined by the skilled artisan. Itis generally preferred that the desublimators are filled with anon-porous material, most preferably having a high surface area to sonot significantly impede gas flow through the desublimator. Structuredtower packing is especially preferable, but random packing mayalternatively be used.

With further reference to FIG. 1, the packing is cooled to a temperaturethat is below the desublimation temperature of the CO₂ (typically at thepressure of the dry cooled flue gas or the precooled dry flue gas)before the start of the desublimation cycle. As the precooled dry fluegas 106 flows up through the packing, it is cooled as it exchanges heatwith the packing, Solid CO₂ desublimates from the gas and collects onthe packing media. The cold CO₂-lean flue gas 107 leaves the firstdesublimator C-101 A, and is routed to a second desublimator C-101 B.The cold CO₂-lean flue gas 107 cools the packing in the seconddesublimator C-101 B and is thereby warmed by the packing media in thesecond desublimator. After a portion of the refrigeration content isrecovered from the CO₂-lean flue gas, the gas exits the seconddesublimator as cool CO₂-depleted flue gas 108. Remaining refrigerationcontent in the cool CO₂-depleted flue gas 108 cools the dry cooled feedgas 104 in exchanger E-102, to form CO₂-depleted gas 109 that furthercools the pressurized flue gas in exchanger E-101 before being vented tothe atmosphere as stack gas 110.

After the solid CO₂ has been deposited in the first desublimator C-101A, the solid CO₂ must be recovered. In the example of FIG. 1, therecovery of previously desublimated CO₂ is performed in a thirddesublimator C-101 C, which in an earlier cycle operated asdesublimator. Here, a high pressure warm liquid CO₂ stream 201 entersthe third desublimator and flows through the packing media. As thepressure in the CO₂ desublimator increases due to contact with thehigh-pressure warm liquid CO₂ stream 201, the solid CO₂ in the thirddesublimator begins to melt. Effluent stream 202 (typically a two-phasestream comprising liquid and solid CO₂) is withdrawn from the thirddesublimator and is heated in the CO₂ liquid heater E-104, where therefrigeration content from the effluent stream 202 (CO₂ slurry) isrecovered. As a result, a combined liquid CO₂ is formed and fed asstream 203 to liquid CO₂ collection drum V-101. Liquid withdrawn fromthe liquid CO₂ collection drum V-101, stream 204, is split into streams205 and 206. Stream 206 enters the circulating liquid CO₂ pump P-102,while stream 205 enters the CO₂ product pump P-101. Stream 206 ispressurized and returned to the desublimator for additional CO₂ melting,as high-pressure warm liquid stream 201. Stream 205 is pressurized andpumped to the CO₂ end-use destination (e.g., storage, sequestration,etc.), as stream 207.

Before the CO₂ is desublimated on the packing media, but after thepacking media has been cooled by the depleted flue gas, the packing iscooled below the desublimation temperature of CO₂ by a refrigerant.While numerous refrigerants are deemed suitable for use herein, it isespecially preferred that the refrigerant comprises dry N₂, O₂, CO₂,air, CO₂ depleted flue gas, or any reasonable combination thereof. Itshould be noted that use of CO₂-depleted flue gas as the refrigerant isespecially advantageous as that gas is already a dry gas (See FIGS. 2Aand 2B, 307 and 308).

Still referring to FIG. 1, cold refrigerant stream 301 enters a fourthdesublimator C-101 D for deep-cooling, which was previously subjected topre-cooling with cold CO₂-lean flue gas. The cold refrigerant cools thepacking material to decrease the temperature in the desublimator toallow for CO₂ desublimation in the next step of the cycle. In turn, thecold refrigerant warms as it pulls heat from the packing material andexits the desublimator as cool refrigerant stream 302. In especiallypreferred aspects of the inventive subject matter, the remainingrefrigeration content in the cool refrigerant 302 is used to cool(typically at least partially condense) chilled pressurized refrigerant305 in exchanger E-103, thus forming warmed refrigerant stream 303. Theso formed warmed refrigerant stream 303 is routed to the refrigerationcompressor K-101 where its pressure is increased to yield warmpressurized refrigerant stream 304. The warm pressurized refrigerantstream 304 is then fed to the liquid CO₂ heater E-104 to thereby heatthe effluent stream and form the chilled pressurized refrigerant 305. Asalready noted above, the chilled pressurized refrigerant 305 is furthercooled in E-103 to form cold pressurized refrigerant stream 306 that isthen expanded in the refrigerant expander T-101 and work is produced.The so formed cold refrigerant stream 301 is routed back to thedesublimator for deep cooling. Of course, it should be recognized thatnumerous alternative refrigeration systems may be used in conjunctionwith the teachings presented herein, and especially suitable systems aredescribed in U.S. Pat. No. 5,483,806 to Miller et al., which isincorporated herein by reference.

Thus, it should be appreciated that the systems and methods of CO₂desublimation of the inventive subject matter provide for heretoforeunprecedented cold integration. In especially preferred aspects,residual refrigeration content of cold CO2-Jean flue gas is used in aprecooling step for a desublimator and flue gas cooling such that thetemperature of the CO2-lean flue gas leaving the plant is no less than0° C., more typically no less than 10° C., even more typically no lessthan 15° C., and most typically no less than 20° C. Additionally, itshould be recognized that refrigeration content of previouslydesublimated CO₂ can be recovered by providing refrigeration duty in aclosed refrigeration cycle that is used to deep-cool a desublimationcolumn. Lastly, at least some of the refrigeration content from theclosed refrigeration cycle can be recycled by heat exchange of the coolrefrigerant stream against the chilled pressurized refrigerant.

Moreover, it should be appreciated that while desublimation is performedat about flue gas pressure, regeneration of the desublimation column isperformed at substantially increased pressure to so allow for a phasetransition of the solid CO₂ to liquid CO₂ (and most preferably at apressure and temperature that allows only for liquid CO₂ and solid CO₂but not gaseous CO₂ to exist).

FIG. 2 depicts a modification of the configuration of FIG. 1 in whichdesublimation process occurs at an elevated pressure, preferably 25-130psia. With respect to the numerals in FIG. 2, the same considerationsand modifications apply to like components which have like numerals.Here BL-101 provides the elevated pressure (e.g., between 20-150 psia)to the flue gas and so increases the desublimation temperature, which inturn reduces work input required by the refrigeration cycle.Consequently, the total power needed per unit of CO₂ captured may besubstantially reduced. In such systems and methods, it should berecognized that the energy used to reach the elevated pressure can berecovered using an expander after the CO₂-lean gas pre-cools adesublimator (stream 108 into the expander). In such configuration, aportion of the CO₂-lean gas may be advantageously used as a refrigerant(via streams 307, 308).

It should be apparent to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refer to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

What is claimed is:
 1. A method of treating flue gas in a flue gastreatment plant, comprising: using a first desublimator to receive aprecooled flue gas, and to produce solid CO₂ and a cold CO₂-lean fluegas using desublimation; using the cold CO₂-lean flue gas to pre-cool asecond desublimator to a temperature above desublimation temperature forCO₂, thereby forming a cool CO₂-depleted flue gas, and using residualrefrigeration content of the cool CO₂-depleted flue gas to cool a feedgas to thereby form the precooled flue gas; wherein the firstdesublimator is deep-cooled using a refrigerant of a refrigeration cyclebefore the step of using the first desublimator; and wherein therefrigeration cycle is thermally coupled to a heat exchanger that coolsthe refrigerant in the refrigeration cycle using refrigeration contentof stream within the flue gas treatment plant.
 2. The method of claim 1,wherein the stream within the flue gas treatment plant is an effluent ofa third desublimator.
 3. The method of claim 2, wherein the effluent isa two-phase stream comprising liquid CO₂ and solid CO₂.
 4. The method ofclaim 1, further comprising a step of removing the solid CO₂ from thefirst desublimator using a liquid CO₂ stream at a pressure andtemperature that does not allow for formation of gaseous CO₂.
 5. Themethod of claim 4, wherein the pressure is between 100-300 psia and thetemperature is between −10 to 40° C.
 6. The method of claim 1, whereinthe first desublimator is operated at a pressure of between 10-50 psia.7. The method of claim 1, wherein the flue gas and a stack gas leavingthe flue gas treatment plant have a temperature of between 10-40° C. 8.A method of recycling refrigeration content in a flue gas treatmentplant sing desublimation, comprising: pre-cooling a flue gas and asecond desublimator using the refrigeration content in a cold CO₂-leanflue gas leaving a first desublimator to which the precooled feed gas isfed; and before operating the first desublimator in a desublimationmode, deep-cooling the first desublimator using a refrigerant of arefrigeration cycle, wherein the refrigerant is cooled by a desublimatoreffluent in the flue gas treatment plant.
 9. The method of claim 8,wherein the step of pre-cooling the flue gas uses two separate heatexchangers, wherein the first heat exchanger cools the feed gas to atemperature above 0° C., and wherein the second heat exchanger cools thefeed gas to a temperature above a desublimation temperature for CO₂. 10.The method of claim 9, further comprising a step of dehydrating the fluegas.
 11. The method of claim 8, wherein the effluent n the flue gastreatment plant is an effluent of a desublimator in regeneration mode.17. A method of treating a flue gas, the method comprising: cooling theflue gas in a heat exchanger using a cool CO₂ depleted flue gas tothereby produce a precooled flue gas at a flue gas pressure; receivingthe precooled flue gas in a first desublimator; producing a coldCO₂-lean flue gas and solid CO₂ in the first desublimator; receiving thecold CO₂-lean flue gas in a second desumblimator; pre-cooling the seconddesublimator with the cold CO₂-lean flue gas to form the cool CO₂depleted flue gas; receiving a liquid CO₂ stream in a thirddesumblimator at a regeneration pressure; producing an effluent streamfrom the third desumblimator; and deep-cooling a fourth desublimator toat least a temperature at which CO₂ desublimates using a refrigerationcycle, wherein the refrigeration cycle is thermally coupled to theeffluent stream of the third desublimator such that a refrigerationcontent of the effluent stream cools a refrigerant in the refrigerationcycle.
 13. The method of claim 12, further comprising: cooling the fluegas using residual refrigeration content of the cool CO₂ depleted fluegas.
 14. The method of claim 12, further comprising: removing water fromthe flue gas.
 15. The method of claim 12, wherein the refrigerationcycle is a closed refrigeration cycle
 16. The method of claim 15,wherein the refrigeration cycle uses a portion of the cool CO₂ depletedflue gas as a refrigerant.
 17. The method of claim 12, wherein therefrigeration cycle includes a cross-heat exchanger that uses arefrigeration content of a stream leaving the fourth desublimator. 18.The method of claim 12, wherein at least one of the first desublimator,the second desublimator, the third desublimator, or the fourthdesublimator comprises a structured packing, a random packing, or anon-porous high surface area material.
 19. The method of claim 12,wherein the flue gas pressure is between 1.0 and 50 psia, and whereinthe regeneration pressure is between 100-300 psia.
 20. The method ofclaim 12, further comprising: switching of an operational mode of atleast one of the first desublimator, the second desublimator, the thirddesublimator, or the fourth desublimator from one of a desublimationmode, a regeneration mode, a pre-cooling mode, and a deep-cooling modeto another one of the desublimation mode, the regeneration mode, thepre-cooling mode, and the deep-cooling mode.